Dual-chamber pacemaker devices have been known and used for a long time. They make it possible to regulate the cardiac activity of the ventricle by using information collected at the level of the atrium of the patient's heart in case of a failure of the atrial-ventricular conduction, which will then induce a block, at the level of the ventricles, of the response to a depolarization of the atriums.
A pacemaker of this kind detects the depolarization of an atrium through what cardiologists designate as the "P" wave, and will then stimulate the ventricle, if this ventricle has not spontaneously depolarized after a given or selected atrialventricular delay. Such a pacemaker also is capable of not stimulating the ventricle in response to "P" waves occurring at too close intervals, to avoid putting a useless or unnecessary strain on the heart.
A dual-chamber pacemaker is, in addition, capable of stimulating the atrium when there is no longer an atrial detection, i.e., a spontaneous atrial depolarization. In a manner known per se, when there is no longer an atrial detection, the pacemaker will stimulate the atrium and, after the atrialventricular delay, it will stimulate the ventricle.
At present, two methods are used in the case of such an absence of atrial detection. In one method, the heart is immediately subjected to a stimulation with a frequency equal to the basic frequency, that is, the minimum frequency which the patient's heart is able to withstand. In some cases, this involves a large change in frequency. In the other method, the stimulation is initially maintained at the stimulation frequency that existed before the absence of atrial detection, for example, for eight beats, and then the frequency is gradually reduced down to the basic frequency. U.S. Pat. No. 4,313,442 discloses a pacemaker in which the stimulation rhythm decreases gradually down to the basic frequency.
The basic frequency is defined as the largest possible value which can be assumed by an escape interval, corresponding to the time at the end of which the atrium must be compulsorily stimulated if the spontaneous depolarization thereof is not detected. This escape interval is regularly recalculated, so that its value tends to approach ever more closely the value separating two successive "P" waves.
A drawback to the known method is that any atrial event, whatever it may be, will be taken into account in the re-calculation of the atrial escape interval (AEI). In the case of frequent fast atrial rhythms, this leads to an increasingly fast rhythm.
At present, it is known to carry out this calculation by measuring the delay between two "P" waves either at each eighth cycle or after each cycle, the cycle being defined as the set of an atrial depolarization and of the next following atrial depolarization. A problem with this method is that the measurements may be made during an atrial extrasystole, that is, a premature spontaneous atrial depolarization. In such case, the delay between the two successive "P" waves is abnormally short. This results in the calculation of a short atrial escape interval and an accelerated stimulation of the atrium, which tends to stimulate the heart at an uselessly high frequency, in the case of an absence of atrial detection.
There is thus a continuing need to provide improved methods for stimulating a patient's heart, particularly the atrium, at appropriate times and rates.